Fabrication process and layout for magnetic sensor arrays

ABSTRACT

A magnetic sensor includes a plurality of groups, each group comprising a plurality of magnetic tunnel junction (MTJ) devices having a plurality of conductors configured to couple the MTJ devices within one group in parallel and the groups in series enabling independent optimization of the material resistance area (RA) of the MTJ and setting total device resistance so that the total bridge resistance is not so high that Johnson noise becomes a signal limiting concern, and yet not so low that CMOS elements may diminish the read signal. Alternatively, the magnetic tunnel junction devices within each of at least two groups in series and the at least two groups in parallel resulting in the individual configuration of the electrical connection path and the magnetic reference direction of the reference layer, leading to independent optimization of both functions, and more freedom in device design and layout. The X and Y pitch of the sense elements are arranged such that the line segment that stabilizes, for example, the right side of one sense element; also stabilizes the left side of the adjacent sense element.

This application claims the benefit of U.S. Provisional Application No.61/438,007 filed 31 Jan. 2011.

TECHNICAL FIELD

The exemplary embodiments described herein generally relates to thefield of magnetoelectronic devices, and more particularly to CMOScompatible magnetoelectronic field sensors used to sense magneticfields.

BACKGROUND

Sensors are widely used in modern systems to measure or detect physicalparameters, such as position, motion, force, acceleration, temperature,pressure, etc. While a variety of different sensor types exist formeasuring these and other parameters, they all suffer from variouslimitations. For example, inexpensive low field sensors, such as thoseused in an electronic compass and other similar magnetic sensingapplications, generally are Hall effect devices with flux concentratorsor anisotropic magnetoresistance (AMR) based devices. In order to arriveat the required sensitivity and reasonable resistances that mesh wellwith CMOS, the sensing units of AMR sensors are generally on the orderof square millimeters in size, while the auxiliary CMOS associated withhall effect sensors can similarly become large and expensive. For mobileapplications, such AMR sensor configurations are too costly, in terms ofexpense, circuit area, and power consumption.

Other types of sensors, such as magnetic tunnel junction (MTJ) sensorsand giant magnetoresistance (GMR) sensors, have been used to providesmaller profile sensors, but such sensors have their own concerns, suchas inadequate sensitivity and being effected by temperature changes. Toaddress these concerns, MTJ, GMR, and AMR sensors have been employed ina Wheatstone bridge structure to increase sensitivity and to eliminatetemperature dependent resistance changes. For minimal sensor size andcost, MTJ or GMR elements are preferred. Typically, a Wheatstone bridgestructure uses magnetic shields to suppress the response of referenceelements within the bridge so that only the sense elements (and hencethe bridge) respond in a predetermined manner. However, the magneticshields are thick and their fabrication requires carefully tuned NiFeseed and plating steps. Another drawback associated with magneticshields arises when the shield retains a remnant field when exposed to astrong (˜5 kOe) magnetic field, since this remnant field can impair thelow field measuring capabilities of the bridge structure. To prevent theuse of magnetic shields, a Wheatstone bridge structure may include twoopposite anti-ferromagnetic pinning directions for each sense axis,resulting in four different pinning directions which must beindividually set for each wafer, very often requiring complex andunwieldy magnetization techniques.

Increasing the number of sensor elements in an array provides a desiredhigher signal to noise ratio. However, the sensor elements must beresettable into a known orientation in the event that they should bescrambled by exposure to an external magnetic field. A reset line maycreate the resetting magnetic field by providing a reset current pulseof short duration. The resistance of the reset line limits the line runlength while allowing pulses of sufficient reset current from a fixedvoltage overhead and therefore limits the array size. Increasing thedensity of the sensor elements within an array through processing oroptimal sensor layout improves the signal to noise ratio without anyadditional complication.

Accordingly, it is desirable to provide a magnetoelectronic sensorfabrication method and layout having a high signal to noise ratio formeasuring various physical parameters. There is also a need for asimple, rugged and reliable sensor that can be efficiently andinexpensively constructed as an integrated circuit structure for use inmobile applications. There is also a need for an improved magnetic fieldsensor and method to overcome the problems in the art, such as outlinedabove. Furthermore, other desirable features and characteristics of theexemplary embodiments will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

A magnetic sensor is configured for enabling independent optimization ofthe material resistance area (RA) of the MTJ and setting total deviceresistance so that the total resistance is not so high that Johnsonnoise becomes a signal limiting concern, and yet not so low that CMOSelements may diminish the read signal. Alternatively, anotherconfiguration results in independent optimization of both functions, andmore freedom in device design and layout. The X and Y pitch of the senseelements may be arranged such that the line segment that stabilizes, forexample, the right side of one sense element; also stabilizes the leftside of the adjacent sense element.

In an exemplary embodiment, a method of fabricating a magnetic sensor,comprises forming a plurality of groups, each comprising a plurality ofmagnetic tunnel junction devices, wherein forming each of the magnetictunnel junction devices comprises forming a synthetic antiferromagneticreference layer; forming a tunnel barrier over the syntheticantiferromagnetic reference layer; and forming and patterning aplurality of sense elements over the tunnel barrier wherein theplurality of sense elements utilize a common shaped reference layer; andforming a plurality of conductors over the magnetic tunnel junctiondevices, wherein the plurality of conductors and the syntheticantiferromagnetic reference layers and are configured to couple one ofthe magnetic tunnel junction devices within each group in parallel andthe groups in series; or the magnetic tunnel junction devices withineach of at least two groups in series and the at least two groups inparallel.

In another exemplary embodiment, a magnetic sensor comprises a pluralityof groups, each group comprising one or more subgroupings of a pluralityof magnetic tunnel junction devices, each subgrouping comprising anelectrode and a shaped reference layer over the electrode; a tunnelbarrier over the shaped reference layer; and a plurality of senseelements over the tunnel barrier layer wherein a sense element, theportion of the tunnel barrier layer under the sense element, and theportion of the reference element under the sense element form a magnetictunnel junction device; and a plurality of conductors over the pluralityof magnetic tunnel junction devices, wherein the plurality of conductorsand one or more of the electrodes are configured to electrically coupleone of the magnetic tunnel junction devices within each group inparallel and the groups in series; or the magnetic tunnel junctiondevices within each of the plurality of groups in series and theplurality of groups in parallel.

In yet another exemplary embodiment, a magnetic sensor comprises aplurality of groups, each group comprising a plurality of magnetictunnel junction devices, each magnetic tunnel junction device within agroup sharing a common synthetic antiferromagnetic reference layer and acommon tunnel barrier positioned on the synthetic antiferromagneticreference layer, and each magnetic tunnel junction device having aunique sense element formed over the tunnel barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 illustrates two active sense elements having magnetizations thatare angled equally in different directions from a pinned layer that willdeflect in response to an externally applied magnetic field and providean output signal related to the component of the magnetic field that isnot aligned with the pinning direction of the pinned layer;

FIG. 2 illustrates an electronic compass structure which usesdifferential sensors formed from two bridge structures with unshieldedMTJ sensors, along with the circuit output for each bridge structure;

FIG. 3 is a simplified schematic perspective view of a Wheatstone bridgecircuit in which series-connected MTJ sensors are aligned to havedifferent magnetization directions from the magnetization direction ofthe pinned layer;

FIG. 4 is a partial schematic perspective view of first and second MTJsensors which include a magnetic field generator structure for clearingor stabilizing the sense layer prior to or during sense operations;

FIG. 5 is a partial cross-sectional view of an integrated circuit inwhich the first and second MTJ sensors shown in FIG. 4 are formed tohave sense layers with different magnetization directions;

FIG. 6 is a simplified schematic top or plan view of a reticle layoutshowing differential sensor formed with a plurality of series-connectedMTJ sensors configured in a Wheatstone bridge circuit with a magneticfield generator structure positioned in relation to the MTJ sensors;

FIGS. 7 and 8 are a partial cross section and a top view, respectively,of a first series of steps in the process of forming a first exemplaryembodiment;

FIGS. 9 and 10 are a partial cross section and a top view, respectively,of a second series of steps in the process of forming a first exemplaryembodiment′

FIGS. 11 and 12 are a partial cross section and a top view,respectively, of a third series of steps in the process of forming afirst exemplary embodiment;

FIGS. 13 and 14 are a partial cross section and a top view,respectively, of a series of steps in the process of forming a secondexemplary embodiment;

FIGS. 15 and 16 are a partial cross section and a top view,respectively, of a second series of steps in the process of formingsecond exemplary embodiment;

FIG. 17 is a perspective view of the second exemplary embodiment;

FIGS. 18 and 19 are a partial cross section and a top view,respectively, of a third series of steps in the process of formingsecond exemplary embodiment;

FIG. 20 is a top view of the third exemplary embodiment;

FIG. 21 is a perspective view of the third exemplary embodiment; and

FIG. 22 is a flow chart of the method in accordance with the exemplaryembodiments.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

A magnetic field sensing device includes, for example at least oneWheatstone bridge, wherein each leg comprises an array of magnetictunnel junctions (MTJs). Each MTJ includes a reference layer, a tunnelbarrier (TB), and a sense layer (or sense element). For the highestsignal to noise ratio in a given chip area (the densest packing of senseelements), groups of these MTJs may share a common reference layer. Theshape of the reference layer may optionally be configured to set areference axis in a first reference direction. A plurality of MTJs(group) are connected in parallel via an upper electrode, and aplurality of these groups are connected in series for (1) enablingindependent optimization of the material resistance area product (RA) ofthe MTJ for best performance; and (2) setting total device resistance sothat the total bridge resistance is not so high that Johnson noisebecomes a signal limiting concern, and yet not so low that CMOS elementsmay diminish the read signal. At least one current line is disposed neareach sensing element. The pitch (distance between or density) of thesense elements is arranged so that the minimal manufacturable senseelement spaces are maintained (˜0.1 um), and current lines near thesense elements may adjoin, yielding a single stabilization strip thattraverses the array. A current source supplies a stabilization currentto the at least one current line, and measuring circuitry is coupled toa sensing element output terminal for measuring the output anddetermining the strength of the magnetic field. An orienting pulse maybe applied to the current line preceding each measurement, or at a fixedor algorithmically determined interval.

In another embodiment, after the pinning step which configures thereference layer magnetization in accordance to its shape is complete,the reference layer may be reshaped, removing portions between specifiedsense elements in the array during the manufacturing method to producethe structure disclosed above. The top electrode is subsequently brokeninto segments of connecting groups of sense elements in series. Thesegroups may be arranged in series or parallel. In this manner, theelectrical connection path and the magnetic reference direction of thereference layer may be individually configured, leading to independentoptimization of both functions, and more freedom in device design andlayout. A larger span of input material resistances may be accommodatedto produce a more precisely controlled final desired device resistance.Additionally, the effect of an individual low resistance element on thetotal bridge output may be diminished dramatically in comparison to itseffect as a member of a parallel wired group as it would short out thatparallel segment. If the array is connected as outlined above, a singlelow resistance element merely reduces the group resistance inapproximate proportion to the total number of array members. Finally,arranging in this series first wiring option reduces the impact of theouter interconnects connecting the groups together on the sensorparameters significantly.

In yet another embodiment, multiple stabilization lines pass proximal toeach sensing element, stabilizing different portions (i.e. left andright sides—but not necessarily limited to two passes of thestabilization line) of each sense element. The X and Y pitch of thesense elements are arranged such that the line segment that stabilizes,for example, the right side of one sense element; also stabilizes theleft side of the adjacent sense element. This configuration allowsfurther flexibility in the arrangement of sense elements within a sensorbridge so that the relative orientations between sense and referencelayer anisotropy axes may be maintained while maximizing the packingdensities of the sense elements. This allows a larger number of senseelements to be stabilized with a fixed line length or conversely a fixedmaximal stabilization line resistance, and at a significantly higherpacking density such that the number of sense elements may be increasedwhile the footprint of the sensor may be reduced.

The sense array is optimally sized for highest possible signal-to-noise(SNR) while allowing the available voltage supply to stabilize thesensors with the required current during the measurement phase. Two ormore copper lines (or parallel connected series of line segments) arerouted adjacent each sense element to maintain the relative orientationsbetween sense and reference layer anisotropy axes while maximizing thepacking densities of the sense elements, thereby allowing for a largernumber of sense elements to be stabilized with a fixed line length or afixed maximal stabilization line resistance. The continuousstabilization line that is formed from the abutting of adjacent cellsstabilizes one section of a sense element in the first pass proximal tothe sense element, and then different sections of the adjacent senseelement. Therefore, each stabilization line segment within a given unitcell is used to stabilize more than one sense element, and asubstantially increased packing density results.

The exemplary embodiments described herein may be fabricated using knownlithographic processes as follows. The fabrication of integratedcircuits, microelectronic devices, micro electro mechanical devices,microfluidic devices, and photonic devices involves the creation ofseveral layers of materials that interact in some fashion. One or moreof these layers may be patterned so various regions of the layer havedifferent electrical or other characteristics, which may beinterconnected within the layer or to other layers to create electricalcomponents and circuits. These regions may be created by selectivelyintroducing or removing various materials. The patterns that define suchregions are often created by lithographic processes. For example, alayer of photo resist material is applied onto a layer overlying a wafersubstrate. A photo mask (containing clear and opaque areas) is used toselectively expose this photo resist material by a form of radiation,such as ultraviolet light, electrons, or x-rays. Either the photo resistmaterial exposed to the radiation, or that not exposed to the radiation,is removed by the application of a developer. An etch may then beapplied to the layer not protected by the remaining resist, and when theresist is removed, the layer overlying the substrate is patterned.Alternatively, an additive process could also be used, e.g., building astructure using the photo resist as a template.

It will be appreciated that additional processing steps will be used tofabricate MTJ sensor structures. As examples, one or more dielectric,ferromagnetic and/or conductive layers may be deposited, patterned andetched using well known techniques, along with conventional backendprocessing (not depicted), typically including formation of multiplelevels of interconnect that are used to connect the sensor structures ina desired manner to achieve the desired functionality. Thus, thespecific sequence of steps used to complete the fabrication of thesensor structures may vary, depending on the process and/or designrequirements.

The disclosed fabrication process may be used to form a magnetic fieldsensor from two differential sensor configurations for a single axisresponse, with only a single pinned direction. For two axis (X, Y)magnetic field response, the sensor only requires two distinct pinningaxes, where each differential sensor is formed from a bridge structureswith unshielded magnetic tunnel junction (MTJ) sensors. For a third axis(Z), no additional pinning direction is required. The distinct pinningaxes may be obtained using shape anisotropy of differently shapedpinning layers in combination with a carefully selected anneal process,or by forming two distinct pinning layers which are separately set andannealed. In a given differential sensor formed from MTJ sensorsconnected in a bridge circuit, shape anisotropy may be used to createsense elements having different magnetizations at zero field that areangled at different orientations from the magnetization of the pinnedlayer, for example, at negative 45 degrees and positive 45 degrees. Inthis configuration, an applied field that includes a component that isorthogonal to the pinning direction will change the magnetization of thedifferent sense layers differently, and as a result, the differentialsensor is able to measure the projection of the applied fieldperpendicular to the pinned axis. The disclosed fabrication process alsoforms a field conductor below and optionally above each MTJ sensor thatmay be used to apply a field pulse along the easy axis of the senselayers to prepare the sensor for measurement, and a smaller current tostabilize the sensor during measurement if desired.

Turning now to FIG. 1, a sensor structure 1 is shown in simplifiedschematic form which uses two active sense element types 20, 30 and apinned layer 10 to measure an external magnetic field. As depicted, themagnetization directions 21, 31 of the active sense elements 20, 30 areangled equally and in different directions from the magnetizationdirection of a pinned layer 10. To this end, the sense elements 20, 30may be formed so that the shape of each sense element is elongated inthe direction of the desired magnetization for that sense element. Thussense elements 20, 30 use their shape anisotropy to create magnetizationdirections that are offset from the pinned layer 10. For example, thefirst sense element 20 may be formed so that its preferred magnetizationdirection is angled at −135 degrees from the magnetization direction ofthe pinned layer 10, and with the second sense element 30 so that itspreferred magnetization direction is angled at 135 degrees from themagnetization direction of the pinned layer 10, although other offsetangles may be used.

Because the conductance across a sense element and pinned layer dependson the cosine of the angle between the sense element and the pinnedlayer, the conductance of the sensor structure can be changed byapplying an external magnetic field (H) which deflects the magnetizationof the sensor elements 20, 30. For example, if there is no applied field(H=0) to a sensor structure 1, then the magnetization directions 21, 31of the sense elements 20, 30 are unchanged, and there is no differencebetween the conductance of the first and second sensor elements 20, 30.And if an external field H is applied to a sensor structure 2 that isdirected along or anti-parallel to the pinned layer 10, the appliedfield will deflect or rotate the magnetic moments 22, 32 of the sensorelements 20, 30 equally, resulting in equal conductance changes for eachsense element, and hence no change in their difference. However, when anexternal field H is applied to a sensor structure 3 that is orthogonalto the pinned layer 10, the magnetic moments 23, 33 for each senseelement 20, 30 are changed differently in response to the applied field.For example, when the external field H shown in FIG. 1 is directed tothe right, the/conductance of the first sense element 20 is increased,while the conductance of the second sense element 30 is reduced,resulting in a difference signal that is related to the field strength.In this way, the depicted sensor structure measures the projection ofthe applied field perpendicular to the pinned axis, but not parallel toit. For low angle magnetization deflections (low field when comparedwith the anisotropy of the sense elements), a Taylor expansion of theresistance, neglecting high order terms, is valid and the resistanceswill also vary in a linear with the conductance. For high angledeflections (higher field strength detection) it is beneficial toutilize a voltage driven, but current detecting half bridge devicetopology to maintain a more linear response.

FIG. 2 shows first and second sensors 201, 211 for detecting thecomponent directions of an applied field along a first x-axis (Axis 1)and a second y-axis (Axis 2), respectively. As depicted, each sensor isformed with unshielded sense elements that are connected in a bridgeconfiguration. Thus, the first sensor 201 is formed from the connectionof sense elements 202-205 in a bridge configuration over a pinned layer206 that is magnetized in a first direction. In similar fashion, thesecond sensor 211 is formed from the connection of sense elements212-215 in a bridge configuration over a pinned layer 216 that ismagnetized in a second direction that is perpendicular to themagnetization direction of the pinned layer 206. In the depicted bridgeconfiguration 201, the sense elements 202, 204 are formed to have afirst magnetization direction and the sense elements 203, 205 are formedto have a second magnetization direction, where the first and secondmagnetization directions are orthogonal with respect to one another andare oriented to differ equally from the magnetization direction of thepinned layer 206. As for the second bridge configuration 211, the senseelements 212, 214 have a first magnetization direction that isorthogonal to the second magnetization direction for the sense elements213, 215 so that the first and second magnetization directions areoriented to differ equally from the magnetization direction of thepinned layer 216. In the depicted sensors 201, 211, there is noshielding required for the sense elements, nor are any special referenceelements required. In an example embodiment, this is achieved byreferencing each active sense element (e.g., 202, 204) with anotheractive sense element (e.g., 203, 205) using shape anisotropy techniquesto establish the easy magnetic axes of the referenced sense elements tobe deflected from each other by 90 degrees.

By positioning the first and second sensors 201, 211 to be orthogonallyaligned with the orthogonal sense element orientations in each sensorbeing deflected equally from the sensor's pinning direction, the sensorscan detect the component directions of an applied field along the firstand second axes. This is illustrated in FIG. 2 with the depicted circuitsimulation shown below each sensor. In each simulation, the simulatedbridge output 207, 217 is a function of an applied field angle for senseelements with an anisotropy field of 10 Oe, applied field of 0.5 Oe, anda magnetoresistance of 100% when the sense element switches from ananti-parallel state to a parallel state. The simulated bridge outputscan be used to uniquely identify any orientation of the applied externalfield. For example, a field that is applied with a 0 degree field angle(e.g., pointing “up” so that it is aligned with the y-axis or Axis 2)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of 10 mV/V from the second sensor 211.Conversely, a field that is applied in the opposite direction (e.g.,pointing “down” so that it is aligned with a 180 degree field angle)will generate a bridge output of 0 mV/V from the first sensor 201, andwill generate a bridge output of −10 mV/V from the second sensor 211.

As seen from the foregoing, a magnetic field sensor may be formed fromdifferential sensors 201, 211 which use unshielded sense elements202-205, 212-215 connected in a bridge configuration over respectivepinned layers 206, 216 to detect the presence and direction of anapplied magnetic field. With this configuration, the possibility ofresidual magnetic moment present in magnetic shielding or NiFe fluxconcentrator, such as present in three dimensional hall devices iseliminated. In addition, the magnetic field sensor provides goodsensitivity, and also provides the temperature compensating propertiesof a bridge configuration. By eliminating the need to form magneticshielding layers, the manufacturing complexity and cost is reduced andthe size of the sensor structure is decreased (in terms of eliminatingthe silicon real estate required to form any shielding layers). Thereare also performance benefits to using unshielded sense elements sincethe magnetic remnance problem is eliminated by removing the magneticshielding and flux guiding layers.

FIG. 3 provides a simplified schematic perspective view of an examplefield sensor 300 formed by connecting four MTJ sensors 301, 311, 321,331 in a Wheatstone bridge circuit, where the series-connected MTJsensors 301, 311, 321, 331 are formed with sense layers 302, 312, 322,332 that are aligned to have different magnetization directions from themagnetization direction of the pinned layers 304, 314, 324, 334. Thedepicted sensor 300 is formed with MTJ sensors 301, 311, 321, 331 thatmay be manufactured as part of an existing MRAM manufacturing processwith only minor adjustments to control the orientation of the magneticfield directions for different layers. In particular, each MTJ sensor301, 311, 321, 331 includes a first pinned electrode 304, 314, 324, 334,an insulating tunneling dielectric layer 303, 313, 323, 333, and asecond sense electrode 302, 312, 322, 332. The pinned and senseelectrodes are desirably magnetic materials, for example, and notintended to be limiting, NiFe, CoFe, Fe, CoFeB and the like, or moregenerally, materials whose magnetization can be collectively aligned.Examples of suitable electrode materials and arrangements are thematerials and structures commonly used for electrodes ofmagnetoresistive random access memory (MRAM) devices, which are wellknown in the art and contain, among other things, ferromagneticmaterials. The pinned and sense electrodes may be formed to havedifferent coercive force or field requirements. The coercive field isbasically the amount of field that is required to reverse the magnetfrom one direction to another after saturation. Technically, it is themagnetic field required to return the magnetization of the ferromagnetto zero after it has been saturated. For example, the pinned electrodes304, 314, 324, 334 may be formed with an anti-ferromagnetic filmexchange coupled to a ferromagnetic film to with a high coercive fieldso that their magnetization orientation can be pinned so as to besubstantially unaffected by movement of an externally applied magneticfield. In contrast, the sense electrodes 302, 312, 322, 332 may beformed with a magnetically soft material to provide different anisotropyaxes having a comparatively low coercive force so that the magnetizationorientation of the sense electrode (in whatever direction it is aligned)may be altered by movement of an externally applied magnetic field. Inselected embodiments, the coercive field for the pinned electrodes isabout two orders of magnitude larger than that of sense electrodes,although different ratios may be used by adjusting the respectivecoercive fields of the electrodes using well known techniques to varytheir composition and/or pinning strength.

As shown in FIG. 3, the pinned electrodes 304, 314, 324, 334 in the MTJsensors are formed to have a first exemplary anisotropy axis alignmentin the plane of the pinned electrode layers 304, 314, 324, 334(identified by the vector arrows pointing toward the top of the drawingof FIG. 3). As described herein, the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 may be obtained using shapeanisotropy of the pinned electrodes, in which case the shapes of thepinned electrodes 304, 314, 324, 334 would each be longer in thedirection of the “up” vector arrow for a single layer pinned magneticstack. In addition or in the alternative, the anisotropy axis alignmentfor the pinned electrodes 304, 314, 324, 334 may be obtained by formingone or more magnetic layers in the presence of a saturating magneticfield that is subsequently or concurrently annealed and then cooled sothat the magnetic field direction of the pinned electrode layers is setin the direction of the saturating magnetic field. As will beappreciated, the formation of the anisotropy axis alignment for thepinned electrodes 304, 314, 324, 334 must be reconciled with thefabrication steps used to form any other field sensors which includepinned electrodes having a distinct anisotropy axis alignment, as wellas any fabrication steps used to form any sense electrodes having adistinct anisotropy axis alignment.

The depicted field sensor 300 also includes MTJ sensors 301, 321 inwhich sense electrodes 302, 322 are formed to have an exemplaryanisotropy axis (identified by the left-pointing vector arrows) that isoffset from the anisotropy axis of the pinned electrodes by a firstdeflection angle. In addition, the depicted field sensor 300 includesMTJ sensors 311, 331 in which sense electrodes 312, 332 are formed tohave an exemplary anisotropy axis (identified by the right-pointingvector arrows) that is offset from the anisotropy axis of the pinnedelectrodes by a second deflection angle which is equal but opposite tothe first deflection angle. In a particular embodiment, the firstdeflection angle is perpendicular to the second deflection angle so thatanisotropy axis of the sense electrodes 302, 322 is rotated 135 degreeswith respect to the anisotropy axis of the pinned electrodes, and sothat anisotropy axis of the sense electrodes 312, 332 is rotatednegative 135 degrees with respect to the anisotropy axis of the pinnedelectrodes.

As will be appreciated, the MTJ sensors 301, 311, 321, 331 may be formedto have identical structures that are connected as shown in series bymetal interconnections in a standard Wheatstone bridge circuitconfiguration with both power supply terminals 341, 343 and outputsignal terminals 342, 344 for the bridge circuit being shown. Byconnecting in series the unshielded MTJ sensors 301, 311, 321, 331 in aWheatstone bridge circuit, the field sensor 300 detects the horizontaldirection (left to right in FIG. 3) component of an externally appliedmagnetic field, thereby forming an X-axis sensor bridge. In particular,a horizontal field component would deflect the magnetization of thesense electrodes 302, 322 differently from the deflection of themagnetization of the sense electrodes 312, 332, and the resultingdifference in sensor conductance/resistance would quantify the strengthof the horizontal field component. Though not shown, a Y-axis sensorbridge circuit may also be formed with unshielded MTJ sensors connectedin a Wheatstone bridge circuit configuration, though the anisotropy axisof the pinned electrodes in the Y-axis sensor bridge circuit would beperpendicular to the anisotropy axis of the pinned electrodes 304, 314,324, 334 in the X-axis sensor bridge. Each of these sensors (or bridgelegs) 301, 311, 321, 331 may represent an array of sense elementsworking in concert to increase the overall SNR of the system.

Low field magnetic sensors are susceptible to Barkhausen noise, sporadicde-pinning, jumps of micro-magnetic domains resulting from differentregions in the magnetic sense element that may have slightly differentorientations of their local magnetic moment from weak localizedmagnetization pinning caused by edge roughness, or small localinhomogeneities in the sense layer, or a myriad of other sources. Suchnoise can introduce errors in accurately measuring the angularresolution of the Earth's magnetic field. When a field is applied, thesemicro-magnetic domains may reverse in a sequential fashion in lieu ofthe desired coherent rotation of the sense element. Prior attempts toaddress such noise have used a hard magnetic bias layer in the senselayers to pin the ends of the device. However, hard bias layers canreduce the sensitivity of the sensor, and have the additionaldisadvantages of requiring an additional processing layer, etch step andanneal step.

To address the Barkhausen noise problem, a magnetic field may beselectively applied along the easy axis of the sense element prior toperforming a measurement. In selected embodiments, the magnetic field isapplied as a brief field pulse that is sufficient to restore themagnetic state of the sense element and remove micro-magnetic domainsthat may have appeared as the result of exposure to a strong field. Inan example implementation, a field pulse is applied to a sensor toremove metastable pinned regions in the sense element, where the fieldpulse has a threshold field strength (e.g., above approximately 40 Oe)and a minimum pulse duration (e.g., of approximately 2-100 nanoseconds).By applying such a field pulse with a predetermined measurement period(e.g., 10 Hz) as required for a compass application, the resulting fieldpulse has an extremely low duty cycle and minimal power consumption. Inaddition, by terminating the field pulse prior to measurement, there isno additional field applied to the sense element during measurement,resulting in maximal sensitivity. Alternatively, a much lowerstabilizing field may be applied through the same reset line duringsensor measurement, minimally impacting the sensitivity, but encouragingclean coherent rotation of the sense element magnetization.

To illustrate an example of how a field pulse may be applied to a senseelement, reference is now made to FIG. 4, which shows a partialschematic perspective view of first and second MTJ sensors 410, 420which each include a magnetic field generator structure 414, 424 forresetting or stabilizing the sense layer 411, 421 prior to or duringsense operations. Each MTJ sensor may be constructed as shown in FIG. 4where the magnetic direction of the sense layer determines theorientation of the magnetic field generator structure. In particular,each MTJ sensor generally includes an upper ferromagnetic layer 411,421, a lower ferromagnetic layer 413, 423, and a tunnel barrier layer412, 422 between the two ferromagnetic layers. In this example, theupper ferromagnetic layer 411, 421 may be formed to a thickness in therange 10 to 10000 Angstroms, and in selected embodiments in the range 10to 100 Angstroms, and functions as a sense layer or free magnetic layerbecause the direction of its magnetization can be deflected by thepresence of an external applied field, such as the Earth's magneticfield. As for the lower ferromagnetic layer 413, 423, it may be formedto a thickness in the range 10 to 2000 Angstroms, and in selectedembodiments in the range 10 to 100 Angstroms, and functions as a fixedor pinned magnetic layer when the direction of its magnetization ispinned in one direction that does not change magnetic orientationdirection during normal operating conditions. As described above, thefirst and second MTJ sensors 410, 420 may be used to construct adifferential sensor by forming the lower pinned layers 413, 423 to havethe same magnetization direction (not shown), and by forming themagnetization direction 415 in upper sense layer 411 to be orthogonal tothe magnetization direction 425 in upper sense layer 421 so that themagnetization directions 415, 425 are oriented in equal and oppositedirections from the magnetization direction of the lower pinned layers413, 423.

To restore the original magnetization of the upper sense layers 411, 421that can be distorted by magnetic domain structure, FIG. 4 depicts amagnetic field generator structure 414, 424 formed below each sensor. Inselected embodiments, the magnetic field generator structure 414, 424 isformed as a conducting current line which is oriented to create amagnetic field pulse which aligns with the magnetization direction 415,425 in the upper sense layer 411, 421. For example, when a current pulseflows through the magnetic field generator structure 414 below the firstMTJ sensor 410 in the direction indicated by the arrow 416, a fieldpulse is created that is aligned with the easy axis 415 of the senseelement 411 in the first MTJ sensor 410. However, since the second MTJsensor 420 has a sense layer 421 with a different magnetizationdirection 425, the magnetic field generator structure 424 is oriented sothat a field pulse is created that is aligned with the easy axis 425 ofthe sense element 421 in the second MTJ sensor 420 when a current pulseflows through the magnetic field generator structure 424 in thedirection indicated by the arrow 426.

The relative alignment of the field pulse and easy axis directions mayalso be seen in FIG. 5, which depicts a partial cross-sectional view ofan integrated circuit device in which the first and second MTJ sensorsshown in FIG. 4 are formed to have sense layers 411, 421 with differentmagnetization directions. In particular, the cross-sectional view on theleft shows the first MTJ sensor 410 as seen from the perspective view 5Ain FIG. 4, while the cross-sectional view on the right shows the secondMTJ sensor 420 as seen from the perspective view 5B in FIG. 4. The firstand second MTJ sensors 410, 420 are each formed over a substrate 430,440 which may have an active circuit 431, 441 embedded therein. On thesubstrate, one or more circuit layers 432, 442 may be formed beforeforming an insulating layer 433, 443 in which a conductive line 414, 424is embedded to form a magnetic field generator structure. As shown inFIG. 5, the conductive line 414 in the first MTJ sensor 410 is formed tocarry current in the direction coming out of plane of the drawing ofFIG. 5, while the conductive line 424 in the second MTJ sensor 420 isformed to carry current moving right-to-left on the drawing. Over theembedded conductive lines, the first and second MTJ cores are formed inan insulating layer 435, 445. In particular, the first MTJ core in thefirst MTJ sensor 410 includes a first conductive line 434 at leastpartially embedded in the insulating layer 435, a lower pinnedferromagnetic layer 413, a tunnel barrier layer 412, an upper senseferromagnetic layer 411 having a magnetization direction 415 that isoriented right-to-left, and a second conductive line 436 over which isformed an additional dielectric layer 437. The first conductive layer434 is connected to a bottom contact layer 438 through a via structure439. In addition, the second MTJ core in the second MTJ sensor 420includes a first conductive line 444 at least partially embedded in theinsulating layer 445, a lower pinned ferromagnetic layer 423, a tunnelbarrier layer 422, an upper sense ferromagnetic layer 421 having amagnetization direction 425 that is oriented into the plane of thedrawing of FIG. 5, and a second conductive line 446 over which is formedan additional dielectric layer 447. To connect the first and second MTJsensors 410, 420, the first conductive layer 444 in the second MTJsensor 420 is connected through a via structure (not shown) to a bottomcontact layer (not shown) in the same level as the embedded conductiveline 424, which in turn is connected through one or more vias andconductive layers to the second conductive line 436 from the first MTJsensor 410. With the depicted configuration, current pulses through theembedded conductive line 414 will create a magnetic field pulse 417which is aligned with the easy axis 415 of the sense element 411, andcurrent pulses through the embedded conductive line 424 will create amagnetic field pulse in the region of the sense element 421 (not shown)which is aligned with the easy axis 425 of the sense element 421.

The lower pinning and pinned ferromagnetic layers 413, 423 may be amaterial, for example, iridium manganese, platinum manganese, cobaltiron, cobalt iron boron, nickel iron, ruthenium, and the like, or anycombination thereof. The tunnel barrier layers 412, 422 may be aninsulating material, for example, aluminum oxide or magnesium oxide. Theupper sense ferromagnetic layers 411, 421 may be a ferromagneticmaterial, for example, nickel iron, cobalt iron, cobalt iron boron,ruthenium, and/or the like. The magnetic field generator structures 414,424 may be aluminum, copper, tantalum, tantalum nitride, titanium,titanium nitride or the like, while conductive lines in general may be,for example, aluminum, copper, tantalum, tantalum nitride, titanium,titanium nitride or the like.

The first and second MTJ sensors 410, 420 may be fabricated together ona monolithic integrated circuit as part of a differential sensor byforming sense layers 411, 421 having orthogonal magnetic orientationsthat each differ equally from the magnetic direction of the pinnedlayers 413, 423. In an example process flow, the first step in thefabrication process is to provide a monolithic integrated circuit chipsubstrate which is covered by a dielectric base layer (not shown). Overthe dielectric base layer, magnetic field generator structures 414, 424are formed as embedded lines of conductive material using knowndeposition, patterning and etching processes so that the magnetic fieldgenerator structures 414, 424 are aligned and positioned below thesensors 410, 420 and embedded in an insulating layer (not shown). Uponthe insulating layer, a stack of sensor layers is sequentially formed bydepositing a first conductive layer (to serve after etching as theconductive line 434), one or more lower ferromagnetic layers (to serveafter etching as the lower pinned ferromagnetic layer 413), one or moredielectric layers (to serve after etching as the tunnel barrier layer412), one or more upper ferromagnetic layers (to serve after etching asthe upper sense ferromagnetic layer 411), and a second conductive layer(to serve after etching as the conductive line 436).

While the various ferromagnetic layers may each be deposited and heatedin the presence of a magnetic field to induce a desired magneticorientation, shape anisotropy techniques may also be used to achieve therequired magnetic orientations for the different ferromagnetic layers.To this end, the sensor layer stack is selectively etched with asequence of patterned etch processes to define the pinned and senselayers in the MTJ sensors 410, 420. In a first etch sequence, the shapesof the different pinning layers 413, 423 are defined from the lowerferromagnetic layer(s) by using patterned photoresist to form a firstpatterned hard mask and then performing a selective etch process (e.g.,reactive ion etching) to remove all unmasked layers down to andincluding the unmasked lower ferromagnetic layer(s). The resultingshapes of the etched lower ferromagnetic layers are oriented so thateach pinned layer has shape anisotropy, resulting in a preferredmagnetic orientation along one of its axes. In addition to being formedas long and narrow shapes, additional shaping of the ends of pinnedlayers may be provided so that each of the pinned layers performs morelike a single magnetic domain. Using shape anisotropy, the shaped pinnedlayers 413, 423 may be annealed to set their respective pinningdirections.

At this point in the fabrication process, the upper ferromagneticlayer(s) will have been selectively etched to leave a remnant portionunder the first patterned hard mask so that the upper and lowerferromagnetic layer(s) have the same shape. However, the final shape ofthe sense layers will be smaller than the underlying pinned layers, andto this end, a second etch sequence is used to define the final shapesof the different sense layers 411, 421 from the remnant portions of theupper ferromagnetic layer(s). In the second etch sequence, anotherphotoresist pattern is used to form a patterned hard mask over the partsof the remnant upper ferromagnetic layer(s) layer that will form thesense layers. The pattern is selected to define high aspect ratio shapesfor the sense layers when a selective etch process (e.g., reactive ionetching) is used to remove all unmasked layers down to and including theunmasked upper ferromagnetic layer(s) 411, 421. In selected embodiments,the selective etch process may leave intact the underlying shaped pinnedlayers 413, 423, though in other embodiments, the selective etch processalso etches the unmasked portions of the underlying shaped pinned layers413, 423. The defined high aspect ratio shapes for the sense layers areoriented so that the sense layers 411 are longer in the dimension of thedesired magnetization 415 than they are wide, while the sense layers 421are longer in the dimension of the desired magnetization 425 than theyare wide. In other words, the long axis for each sense layer is drawnalong the desired magnetization direction for a single ferromagneticsense layer. In addition to being formed as long and narrow shapes,additional shaping of the ends of sense layers 411, 421 may be providedso that each of the sense layers performs more like a single magneticdomain. For example, the sense layers may be shaped to have pointed endsthat taper in the corresponding directions of desired easy axis for thesense layers. Once the shaped sense layers are formed, the desired easyaxis magnetic orientations may be induced from their shape anisotropy bybriefly annealing the wafer (e.g., at an anneal temperature ofapproximately 250 degrees C. in the absence of a magnetic field toremove material dispersions. Upon cooling, the magnetizations of thesense layers 411, 421 align with the individual pattern, providingmultiple orientations of sense layers.

In a practical deployment, the magnetic field generator structures 414,424 are formed from the same layer that is necessary to interconnect thebridge legs, and hence creates no additional processing steps. Inaddition, each of the magnetic field generator structures 414, 424 maybe constructed from a single conductive element that is positioned topass beneath each MTJ sensor with the appropriate orientation, therebycreating field pulses throughout the chip with a single current pulse.While in a practical deployment, each bridge leg will comprise arrays ofsense elements for highest Signal to Noise Ratio (SNR), a simplifiedexample a single sense element implementation is illustrated with FIG. 6which provides a simplified schematic top or plan view of a reticlelayout showing differential sensor 600 formed with a plurality ofseries-connected MTJ sensors 621, 622, 623, 624 configured in aWheatstone bridge circuit with a magnetic field generator structure 620positioned in relation to the MTJ sensors. The depicted differentialsensor includes four pinned layers 601, 602, 603, 604 which each havethe same magnetization direction (e.g., a pinned axis in they-direction), as shown by the large vector arrow on each pinned layer.While the pinned layers 601, 602, 603, 604 may be formed using theirshape anisotropy (as indicated in FIG. 6), they may also be formed usinga traditional field-anneal process.

FIG. 6 also shows that two of the MTJ sensors or sensor arrays, 621, 624in the differential sensor are formed with sense layers 611, 614 havinga magnetization direction that is oriented at negative 135 degrees fromvertical, as shown in the sense layers 611, 614. The other two MTJsensors 602, 603 are formed with sense layers 612, 613 having amagnetization direction that is oriented at 135 degrees from vertical,as shown in the sense layers 612, 613. While any desired technique maybe used to form the sense layers having different magnetizationdirections, selected embodiments of the present invention use shapeanisotropy techniques to shape the sense elements 611, 614 to have amagnetization direction (or easy axis) that is oriented at predetermineddeflection angle from vertical, and to shape the sense elements 612, 613to have a magnetization direction (or easy axis) that is orientednegatively at the predetermined deflection angle from vertical. In thisway, the magnetization direction of the sense elements 611, 614 and themagnetization direction of the sense elements 612, 613 are offsetequally in opposite directions from the magnetization direction of thepinned layers 601, 602, 603, 604.

The depicted differential sensor 600 also includes a magnetic fieldgenerator structure 620 which is formed beneath the MTJ sensors 621,622, 623, 624 so as to selectively generate a magnetic field tostabilize or restore the magnetic field of the sense layers 611, 612,613, 614. In selected embodiments, the magnetic field generatorstructure 620 is formed as a single conductive line which is arranged tocarry current beneath the sense layers 611, 612, 613, 614 in a directionthat is perpendicular to the easy axis orientation of the sense layersso that the magnetic field created by the current is aligned with theeasy axis. Thus, the conductive line 620 is formed below the fourth MTJsensor 624 to create a magnetic field that is aligned with the easy axisof the sense element 614. In addition, the orientation of the conductiveline 620 below the second and third MTJ sensors 622, 623 creates amagnetic field that is aligned with the easy axis of the sense elements612, 613. Finally, the conductive line 620 is formed below the first MTJsensor 621 to create a magnetic field that is aligned with the easy axisof the sense element 611.

An exemplary fabrication design and layout for forming the abovedescribed sensors 410, 420 (FIG. 4) and the magnetic field generatorstructures 414, 424, 620 in three array embodiments is described inFIGS. 7-21. A substrate (not shown), in which the structures describedbelow are integrated, may be implemented as a bulk silicon substrate,single crystalline silicon (doped or undoped), or any semiconductormaterial including, for example, Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs,InP as well as other Group III-IV compound semiconductors or anycombination thereof, and may optionally be formed as the bulk handlingwafer. In addition, the substrate may be implemented as the topsemiconductor layer of a semiconductor-on-insulator (SOI) structure.Though not shown, one or more circuit elements may be formed on or inthe substrate. In addition, a base insulator layer (not shown) is formedby depositing or growing a dielectric (e.g., silicon dioxide,oxynitride, metal-oxide, nitride, etc.) over the semiconductor substrateusing chemical vapor deposition (CVD), plasma-enhanced chemical vapordeposition (PECVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), ion beam deposition (IBD), thermal oxidation, orcombinations of the above.

Referring to the partial cross sectional view of FIG. 7 and the top viewof FIG. 8, the stack of sensor layers is sequentially formed bydepositing a first conductive layer (702) to serve as a lower electrode,one or more pinning layers 704 to pin the lower pinned ferromagneticlayer, a synthetic antiferromagnetic structure (SAF) 706, one or moredielectric layers 708 to serve, after etching, as the tunnel barrierlayer, one or more upper ferromagnetic layers 710 to serve, afteretching, as the upper sense ferromagnetic layer. Each of the layers maybe blanket deposited to a predetermined thickness using knowntechniques, such as CVD, PECVD, PVD, ALD, IBD, or combinations thereof.In this way, the stack of sensor layers covers the entire wafer so thatthe stack is formed in the “Sensor 1” area where a first type of sensor(e.g., x-axis sensors) will be formed, and is also formed in the “Sensor2” area where a second type of sensor (e.g., y-axis sensors oriented 90degrees from the “Sensor 1”) will be formed. In addition, the sensorstack may be formed in “other” areas where a sensor having any desiredorientation will be formed.

The electrode 702 is any conductive material such as Cu or Al, butpreferably is Ta, TaN or a combination thereof. The pinningantiferromagnetic layer, 704, preferably is PtMn. The SAF 706 may be twoferromagnetic layers (not shown) separated by a non-magneticantiferromagnetic coupling spacer (not shown) as is well known by thoseskilled in the art. The tunnel barrier 708 preferably comprises AlOx orMgO.

As will be appreciated, SAF structures, as in SAF 706, with appropriategeometry and materials that provide sufficiently strong (hard)magnetization, the underlying anti-ferromagnetic pinning layer may notbe required, thereby providing a simpler fabrication process with costsavings. For example, a slightly imbalanced SAF (not shown) having twodiffering ferromagnet layers separated by a Ruthenium spacer layer,where the ferromagnetic layers above and below the ruthenium layer havedifferent magnetic moments. Either or both of the ferromagnetic layersmay be formed with CFB (Cobalt Fe Boron) or any desired ferromagneticalloy. For example, CoFe may be used for the lower layer and CFB may beused for the upper layer in an example embodiment. At certain periodicthicknesses of the Ruthenium spacer layer, the two ferromagnet layerswill be exchange coupled so that the anti-parallel state is the lowenergy state. As a result, the net magnetic moment is minimized and theimmunity to external field response is strengthened. With this exampleSAF structure, a small net moment may be created which will respond to avery large externally applied magnetic field before the SAF is pinnedvia a high temperature anneal. For a reference layer formed with a SAFthat has micron scale dimensions (e.g., greater than (approximately 0.5um along the short axis and a high aspect ratio of greater than about3), the magnetization tends to align anti-parallel along the short axisinstead of along the long axis, hence the short axis sets the pinningdirection. This results from the fact that the lowest energy state isfor the two layers to close their magnetic flux along the short axis ofthe patterned shape. In remanence (e.g., after the setting field isremoved), the magnetic moment of the largest moment layer (e.g., thelower pinned layer of 706, FIG. 7 in this example) aligns so that it isalong the short axis of the SAF in the direction that has a positiveprojection onto the setting field angle (see U.S. Patent Publication2009/0279212, assigned to the assignee of the present invention).Conversely, the magnetic moment of the smaller moment layer (e.g., theupper fixed or reference layer in this example) aligns in the oppositedirection from the pinned layer.

The desired pinning direction for the reference layers may be induced byfirst heating or annealing the wafer below the phase transitiontemperature of the PtMn in the presence of a saturating field that isoriented between the orientations of the reference layers 706.Subsequently, the saturating field is removed, the temperature isbrought above the phase transition temperature to pin the SAF and fixthe ferromagnetic reference layer. In a zero or small compensatingmagnetic field, the magnetic orientation of the reference layer isaligned parallel to the short axis of the patterned magnetic referencelayer. By heating the wafer above the phase transition temperature in azero or small compensating field, the respective shape of the referencelayers 706 provides shape anisotropy which in conjunction with thecompensating field causes the magnetization of each reference layer torelax and pin along a desired direction. Thus, the magnetization ofreference layer 706 follows its shape so that it is aligned parallel tothe short axis of the patterned shape. Next, sensor elements 910 (FIGS.9 and 10) are shaped from the sensor layer 710 by a reactive ion or etchor sputter etch, for example. In this second etch sequence, anotherphotoresist pattern is used to form a patterned hard mask over the partsof the remnant upper ferromagnetic layer(s) layer that will form thesense elements 910. The pattern is selected to define high aspect ratioshapes for the sense elements 910 when a selective etch process (e.g.,reactive ion etching) is used to remove all unmasked layers down to andincluding the unmasked upper ferromagnetic layer 710. In selectedembodiments, the selective etch process may leave intact the underlyingshaped pinned layers 706, though in other embodiments, the selectiveetch process also etches the unmasked portions of the underlying shapedpinned layers 706. The defined high aspect ratio shapes for the senseelements 910 are oriented so that they are longer in the dimension ofthe desired magnetization than they are wide. In other words, the longaxis for each sense element 910 is drawn along the desired magnetizationdirection for a single ferromagnetic sense layer. In addition to beingformed as long and narrow shapes, additional shaping of the ends ofsense elements may be provided so that each of the sense elementsperforms more like a single magnetic domain. For example, the senseelements may be shaped to have pointed ends that taper in thecorresponding directions of desired easy axis. Once the shaped senseelements are formed, the desired easy axis magnetic orientations may beinduced from their shape anisotropy by briefly annealing the wafer(e.g., at an anneal temperature of approximately 250 degrees C.). in theabsence of a magnetic field to remove material dispersions. Uponcooling, the magnetizations of the sense layers 910 align with theindividual pattern, providing multiple orientations of sense layers.This brief anneal may be included as part of the subsequent waferprocessing such as a pre-CMP copper anneal for upper interconnectlayers.

A plurality of conductors 1141, 1142 are deposited (FIGS. 11 and 12),one each over each of two groups of sense elements 1116, 1117, therebycoupling the magnetic tunnel junction devices 1116 within a group inparallel between the conductors 1141-1142 and the electrode 702.

These parallel coupled sense elements 1116 may be coupled in series withthe groups 1117 by conductor 1151.

This embodiment enables many sense elements to be connected in parallelbetween the electrodes, allowing noise averaging at the device levelbetween the sense elements, increasing the SNR. Since multiple senseelements are stacked on a single reference electrode, they may be packedmuch more densely than would be otherwise possible given the designrules for sense element to lower electrode overlay and lower electrodespacing.

In a second exemplary embodiment, referring back to FIGS. 7-10, achemical etch is performed through the sense layer 710, the tunnelbarrier 708, the SAF 706, and at least to the pinning layer 704, tocreate an opening 714 (FIG. 8), but could be partially or completelythrough the pinning layer 704. The pinned reference layer 704 is formedwith a single patterned ferromagnetic layer having a magnetizationdirection 703 (FIG. 8) that aligns along the short-axis of the patternedreference layer(s). The synthetic anti-ferromagnetic (SAF) layer 706aligns the magnetization of the pinned reference layer along the shortaxis of the patterned reference layer(s). In a first etch sequence, theshapes of the different pinning layer 704 are defined by using patternedphotoresist to form a first patterned hard mask and then performing aselective etch process (e.g., reactive ion etching) to remove allunmasked layers down to and including the unmasked lower ferromagneticlayer(s). The resulting shapes of the etched lower ferromagnetic layersare oriented so that each pinned layer has shape anisotropy, resultingin a preferred magnetic orientation along one of its axes.

The resulting shapes of the etched lower ferromagnetic layers areoriented so that each pinned layer has shape anisotropy, resulting in apreferred magnetic orientation along one of its axes as disclosed above.Once the magnetic reference angle has been set, it is not necessary topreserve the high aspect ratio of the pinned magnetic layer since themagnetization is no longer free to move. As the pinned layer and lowerelectrode also serve as the electrical interconnection path, when onlyconsiderations of its magnetic behavior are taken into account in thedevice design, the optimal sense element density and device performancemay not be realized. When the tunnel junctions are further separatedinto groups via subsequent patterning of the lower electrode, very closeto the ideal electrical pattern and packing densities may also beutilized. The shape dependent magnetic function to determine sensorreference angle of the lower electrode has already been performed.

Then subsequent to the sense elements 910 and dielectric layer 708 beingetched to define the sense element shapes, a further chemical etch(FIGS. 13-14) is performed through the SAF 706, the pinning layer 704,and the lower electrode to create openings 1314 and create subgroups1331-1338 of sense elements. These subgroups are connected together ingroups (FIG. 15-16) for ideal electrical performance by interconnectionwith optional vias 1544 and an upper local interconnect layer 1542. Aplurality of conductors 1561-1568 are formed over certain of the sensorelements 910, coupling a sense element from one subgroup to a senseelement of another subgroup in series. For example, conductor 1561couples a sense element 910 in subgroup 1531 to a sensor element 910 insubgroup 1532. This structure couples the magnetic tunnel junctions 1016in subgroups 1531, 1532, 1533, 1534 in series, and the subgroups 1535,1536, 1537, 1538 in series. These series coupled subgroups 1531-1538 maythen be coupled in parallel by the conductor 1571. FIG. 17 shows aperspective view of the sense element array.

Referring to FIGS. 18 and 19, a plurality of conductive stabilizationlines 1802 are formed near the sense elements 910. As shown, theconductive stabilization lines 1802 are preferably formed on two opposedsides of the sense elements 910, but may be formed on only one side. Toprepare the sense elements for field measurement, an orienting fieldpulse is applied along the stabilization path. This reset may beperiodic, precede each measurement, or only occur when an errorcondition (very high bridge offset indicating misorientation, linearityerrors, or high noise condition) is encountered. As the individual senseelement anisotropy is large compared with the stabilization field thatmust be applied for noise optimization but on the same order as thefield required to reorient the sense element, the magnitude of theorientation field pulse is much larger than that applied during themeasurement phase for sensor stabilization. Without hard biasstabilization or a measurement preparation capability, a momentaryexposure to a large field may reorient the magnetization of the senseelements in a poorly determined state. Subsequent to the preparationphase (application of the stabilization current), all line segmentswithin a bridge or sense axis are connected in series, and stabilizationcurrent is applied to these segments and a measurement may proceed.

This combination of series and parallel wiring provides the highestsense element packing density, while simultaneously allowing freedom tocreate a device with arbitrary final resistance to provide optimalcombination of device parameters such as impedance for minimal powerconsumption, low Johnson and shot noise, and best CMOS impedancematching. The inner series wiring is further beneficial in that a lowresistance sense element will not contribute too greatly to the overalldevice resistance as its impact is diminished by the other seriesconnected members. Such is not the case with the inner parallel wiringrequired for a single patterning of the lower electrode.

FIGS. 20 and 21 illustrate a third exemplary embodiment wherein themagnetic tunnel junctions 2016 are positioned between conductive lines2002 and 2042 and the adjacent stabilization lines 2004. The conductiveline 2042 is coupled to the sense elements 2016 by a via 2018, and thesense array is optimally sized for highest possible signal-to-noise(SNR) while allowing the available voltage supply to stabilize thesensors with the required current during the measurement phase. Two ormore copper lines (or parallel connected series of line segments) arerouted adjacent each sense element to maintain the relative orientationsbetween sense and reference layer anisotropy axes while maximizing thepacking densities of the sense elements, thereby allowing for a largernumber of sense elements 2016 to be stabilized with a fixed line lengthor a fixed maximal stabilization line 2004 resistance. Taken alone, thearray pitch of the line segments 2004 and the sense elements 2016 arenot identical, but take the relationship that there are multiple senseelements 2016 and stabilization line segments 2004 per unit cell(magnetic tunnel junction) 2020. When this cell 2020 is arrayed out, thecontinuous stabilization line 2004 that is formed from the abutting ofadjacent cells 2020, stabilizes one section of a sense element 2016 inthe first pass proximal to sense element 2016, and then differentsection of the adjacent (proceeding along the stabilization line 2004routing direction) sense element 2016. Therefore, each stabilizationline segment 2004 within a given unit cell 2020 is used to stabilizemore than one sense element 2016, and a substantially increased packingdensity results. Therefore, a higher number of sense elements 2016 maybe placed within a fixed die space, or the sensor die size may befurther reduced while the signal to noise ratio is simultaneouslyincreased.

FIG. 22 is a flow chart that illustrates a process of fabricating theexemplary embodiment. It should be appreciated that process 2200 mayinclude any number of additional or alternative tasks, the tasks shownin FIG. 22 need not be performed in the illustrated order, and process2200 may be incorporated into a more comprehensive procedure or processhaving additional functionality not described in detail herein.Moreover, one or more of the tasks shown in FIG. 22 could be omittedfrom an embodiment of the process 2200 as long as the intended overallfunctionality remains intact.

The process 2200 of fabricating a magnetic sensor includes forming 2202a plurality of groups, each comprising a plurality of magnetic tunneljunction devices, wherein forming each of the magnetic tunnel junctiondevices comprises forming and patterning 2204 a reference elementcomprising forming an electrode and forming a reference layer over theelectrode, forming a tunnel barrier over the reference layer, andforming and patterning a plurality of sense elements over the tunnelbarrier; and forming 2206 a plurality of conductors over the magnetictunnel junction devices, wherein the plurality of conductors and theplurality of electrodes are configured 2208 to electrically couple oneof the magnetic tunnel junction devices within each group in paralleland the groups in series; or the magnetic tunnel junction devices withineach of at least two groups in series and the at least two groups inparallel.

By now it should be appreciated that there has been provided a magneticfield sensor apparatus and a method of manufacturing a plurality ofdifferential sensor circuits over a substrate which detects an appliedmagnetic field directed along a one or more axis. The differentialsensor circuits may be constructed as Wheatstone bridge structures, onearray of sense elements for each axis to be sensed, of unshieldedmagnetic tunnel junction (MTJ) sensors formed with a plurality of pinnedlayers that are each magnetized in a single pinned direction and acorresponding plurality of unshielded sense layers. In an exampleimplementation, the differential sensor circuit includes a firstunshielded MTJ sensor having a first unshielded sense layer with a firsteasy axis magnetic orientation, and a second unshielded MTJ sensorhaving a second unshielded sense layer with a second easy axis magneticorientation, where the first and second easy axis magnetic orientationsare deflected equally and in opposite directions (e.g., ±135 degrees)from the single pinned direction. When each unshielded sense layer isformed to have an anisotropic axis with a longer length dimension and ashorter width dimension, the longer length dimension is aligned with aneasy axis magnetic orientation for the unshielded sense layer. Inselected embodiments, multiple sense elements are positioned on eachpatterned reference layer strip, allowing for closer sense element pitchand higher aspect ratio of the reference layer; hence better magneticcharacteristics of that reference layer for a remnant field settingprocess. In another embodiment, after the remant magnetic field settingprocedure is complete and the high aspect ratio, narrow (1-3 um)reference layer has been utilized, the sense elements are patterned andthen the reference layer is reshaped and cut into segments to allow aseries wiring of the sense elements in conjunction with an upperconducting layer. Each magnetic field sensor includes an embeddedmagnetic field generator disposed near each unshielded sense layer thatis positioned to generate a magnetic field pulse that is aligned with aneasy axis magnetic orientation for each unshielded sense layer. Inselected embodiments, the embedded magnetic field generator isimplemented as a conductive line positioned to conduct a current pulsethat creates a magnetic field pulse for resetting a magnetic orientationof an associated unshielded sense layer, and/or to apply a weak magneticfield along an easy axis magnetic orientation for each unshielded senselayer. In selected embodiments, the X and Y pitch of the sense elementsare arranged such that the line segment that stabilizes, for example,the right side of one sense element; also stabilizes the left side ofthe adjacent sense element. The sense layers within each magnetic fieldsensor are grouped, wherein each group sequentially receives a currentpulse. By providing a current pulse sequentially to these groups, theline resistance is reduced, allowing for a larger current with the givenvoltage.

Although the described exemplary embodiments disclosed herein aredirected to various sensor structures and methods for making same, thepresent invention is not necessarily limited to the example embodimentswhich illustrate inventive aspects of the present invention that areapplicable to a wide variety of semiconductor processes and/or devices.Thus, the particular embodiments disclosed above are illustrative onlyand should not be taken as limitations upon the present invention, asthe invention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. For example, the relative positions of the sense andpinning layers in a sensor structure may be reversed so that the pinninglayer is on top and the sense layer is below. Also the sense layers andthe pinning layers may be formed with different materials than thosedisclosed. Moreover, the thickness of the described layers may vary.Accordingly, the foregoing description is not intended to limit theinvention to the particular form set forth, but on the contrary, isintended to cover such alternatives, modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims so that those skilled in the art shouldunderstand that they can make various changes, substitutions andalterations without departing from the spirit and scope of the inventionin its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

What is claimed is:
 1. A magnetic sensor comprising at least one array,each array comprising: a plurality of groups, each group comprising oneor more subgroupings of magnetic tunnel junction devices, eachsubgrouping comprising: a shaped reference element comprising: anelectrode; and a shaped reference layer over the electrode, a tunnelbarrier layer over the shaped reference layer; and a plurality of senseelements over the tunnel barrier layer, wherein a sense element, theportion of the tunnel barrier layer under the sense element, and theportion of the shaped reference element under the sense element form amagnetic tunnel junction device; and a plurality of conductors over theplurality of magnetic tunnel junction devices, wherein the plurality ofconductors and one or more of the electrodes are configured toelectrically couple one of: the magnetic tunnel junction devices withineach group in parallel and the groups in series; or the magnetic tunneljunction devices within each of the plurality of groups in series andthe plurality of groups in parallel.
 2. The magnetic sensor of claim 1wherein the plurality of conductors comprises: a first conductorelectrically coupled to a first plurality of magnetic sense elementswithin a first plurality of groups; a second conductor electricallycoupled to a second plurality of magnetic sense elements within a secondplurality of groups; and a third conductor electrically coupled to thefirst and second conductor.
 3. The magnetic sensor of claim 1 whereinthe plurality of conductors comprises: a first conductor electricallycoupled to at least one magnetic sense element within a first pluralityof groups; a second conductor electrically coupled to at least onemagnetic sense element within a second plurality of groups; and a thirdconductor electrically coupled to the first and second conductor.
 4. Themagnetic sensor of claim 1 wherein each magnetic tunnel junction devicecomprises a first portion extending in a direction and a second portionextending in an opposed direction, further comprising: first and secondstabilization conductors adjacent to, and configured to stabilize, themagnetic tunnel junction devices, wherein the first stabilizationconductor alternates between the first and second portions of adjacentmagnetic tunnel junction devices, and the second stabilization conductoralternates between the other of the first and second portions ofadjacent magnetic tunnel junction devices.
 5. The magnetic sensor ofclaim 1, further comprising: at least two stabilization line segmentsadjacent each of the plurality of sense elements, thereby configured tomaintain a relative orientation between a magnetic sense layeranisotropy axis and a shaped reference layer anisotropy axis andstabilize at least two magnetic sense elements.
 6. The magnetic sensorof claim 1 further comprising first and second arrays comprising firstand second pluralities of groups, respectively, are electrically coupledin series and third and fourth arrays comprising third and fourthpluralities of groups are electrically coupled in series and the firstand second arrays are electrically coupled in parallel with the thirdand fourth arrays forming a Wheatstone bridge.
 7. The magnetic sensor ofclaim 1 further comprising first and second arrays comprising first andsecond pluralities of groups, respectively, are electrically coupled inseries forming a half Wheatstone bridge.
 8. The magnetic sensor of claim7 further comprising a voltage source connected at a coupling pointbetween the two arrays, a virtual ground of a current preamplifiercoupled at a first side of one array, a virtual ground of a secondcurrent preamplifier coupled at a second side of the other array, thesensor signal consisting of the current difference measured between thetwo current preamplifiers.
 9. The magnetic sensor of claim 1 wherein theshaped reference element comprises a synthetic antiferromagneticelement.